Some forms of high intensity discharge lamps, e.g., high pressure sodium lamps or metal halide lamps (e.g., Pat. Nos. 3,885,184; 3,911,308; 4,409,517), employ ceramic arc tubes constructed of polycrystalline alumina. These tubes have tungsten electrodes welded to niobium feedthrough which are sealed in a sealing disc of alumina; the discs, in turn, being sealed into the ends of the tube by means of a frit. Electrical connection is made to the ends of the feedthroughs which project from the sealing discs. This construction requires multiple hermetic seals and is quite costly and difficult.
It has been proposed that the feedthroughs be made of a conductive cermet having a tungsten electrode mounted therein. The cermet then serves as the electrical contact for the arc tube. See, e.g., U.S. Pat. No. 4,155,758. This procedure still requires some means for mounting the electrode within the feedthrough.